The present invention relates to a video camera and, more particularly, to apparatus and method for emphasizing an outline of a video signal for use in a video camera as well as other video electronic equipment.
As is known, professional video cameras perform various processing of a generated video signal including emphasizing the outlines of images represented therein so as to produce a high quality video output signal. In such video cameras, a subject image is photographed by CCD solid state imaging sensors to produce R (red), G (green) and B (blue) image signals which are then digitized to produce respective R, G and B digital video signals and an outline emphasis signal is produced therefrom that is subsequently added to the video signals. The outline emphasis signal generally increases in size as the level variation of the original signals increase and, thus, the generated outline emphasis signal is relatively large at the image edge points of bright images appearing against a dark background. Subsequently, the addition of the outline emphasis signal to the original signals for bright images produces a resultant image signal having large values at the edges thereof.
Video cameras typically have a dynamic range from -7% on the "black side" to 109% on the "white side", and, as is well-known known, signal levels near the two ends of the dynamic range are clipped during processing and recording thereof. Consequently, very large signal levels of an image signal produced from the addition of the outline emphasis signal thereto also are clipped thus causing the deleterious effect of widening the edges of the images represented by the processed video signal.
Various techniques have been proposed for solving the above-mentioned problem. For example, Japanese laid-open patent application No. 6-46444 proposes to lower the level of the outline emphasis signal when the resultant summed image signal and outline emphasis signal exceeds the dynamic range of the camera. Another technique, proposed in Japanese laid-open patent application 3-277089, suggests to suppress (i.e., not use) the outline emphasis signal for skin colored portions of the image. A further technique, disclosed in Japanese laid-open patent application 6-54232, suggests to add an outline emphasis signal having a low clock rate to the image signal both before and after gamma knee correction for signals having or requiring a relatively low resolution, and to add an outline emphasis signal having a high clock rate to a brightness signal having its clock rate increased using a matrix circuit and the image signal after gamma knee correction when high resolution images are required.
Video cameras currently are known to produce high resolution image signals by spatially shifting by one-half a picture element the R and B image signals horizontally relative to the G image signal, such as previously disclosed in U.S. Pat. No. 5,521,637 and Japanese laid-open patent applications 6-217326 and 6-153217. Still further, it is known to prevent beat interference by coinciding the CCD imaging devices' clock rates to the clock rate of the digitized video signals, such as disclosed in Japanese patent Nos. 6-217326 and 6-153217.
Moreover, Japanese laid-open patent applications 6-217326 and 6-153217 disclose processing the video signals at twice the clock rate of the CCD imaging devices when the aforementioned spatial picture element shift technique is utilized. It also is known to vary the level of a generated outline emphasis signal as a function of the level of the input signal. For example, given an input signal such as shown in FIG. 1A of the drawings and a "gain" characteristic function as shown in FIG. 1B, an output signal as shown in FIG. 1C is produced.
One problem encountered in video cameras and other video processing devices, such as previously discussed, is their general inability to correctly process "burst-like" signals. Namely, outline emphasized signals (i.e., video signals having outline emphasis signals added thereto) that exceed the dynamic range of the camera are uniformly reduced at both the edges of the image signal and the burst-like portions thereof.
Another problem with the aforementioned video cameras is that they generally do not process video signals that are produced from CCD imaging devices having different clock rates. A further problem of existing devices and techniques for gain-adjusting the outline emphasis signal, such as that disclosed with reference to FIGS. 1A to 1C, is that they tend to improperly gain-adjust a spurious high level input signal to produce an exceedingly high output signal that is substantially larger than the adjacent non-spurious signals.